This invention generally relates to centrifugal compressors, and more particularly to methods for controlling and detecting surge within a centrifugal compressor.
Surge occurs when the flow through the compressor is suddenly reduced. The process of surge within a centrifugal compressor results in transient unstable flow of a working fluid within the centrifugal compressor during the surging process. Typically, surge is accompanied by loud objectionable noises, and surge may cause structural damage and fatigue to the centrifugal compressor, either initially or during the life of the centrifugal compressor. More particularly, surge may cause the following to occur to a centrifugal compressor: damage to centrifugal impellers, overheating and overloading of compressor bearings which may cause bearing destruction, fatigue and overheating of centrifugal compressor piping, and overheating of other compressor components.
Presently, there exist centrifugal compressor control systems which are "reactive" to a surge process, and which merely unload the centrifugal compressor after surge occurs. However, in such compressor control systems, the initial stages of surge have already occurred by the time corrective action is taken. Additionally, in order for these control systems to function, the centrifugal compressor is nearly completely unloaded which thereby reduces the fluid output of the associated centrifugal compressors.
Prior art compressor control systems have employed numerous methods for detecting surge. A method used in prior art devices includes the use of a flow meter, such as a venturi nozzle, which is placed in a compressor discharge conduit with appropriate signals to the control system to indicate if a flow reversal has occurred. This method typically is costly, inaccurate, and causes additional system pressure losses. Another method employed in the prior art is premised upon the rate of rise of temperature of the working fluid at the inlet conduit to a centrifugal compressor stage. This method typically responds slower than desired.
The foregoing illustrates limitations known to exist in present centrifugal compressor surge control systems. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.